Typical internal combustion engines include a reciprocating piston disposed within a cylinder having a closed end. A variable volume created between the piston, the cylinder, and the closed end encloses and compresses a fluid, which can include air, a mixture of air and exhaust gas, a combustible mixture, and other fluids. During operation, combustion of the fuel/air mixture creates hot and expanding exhaust gases, which push the piston along the cylinder. The piston is connected to a crankshaft, which includes an offset lobe connected to the piston via a connecting rod such that power generated during combustion pushes the piston, which in turn causes the crankshaft to rotate and produce useable power.
In turbocharged engines a performance tradeoff exists between turbo performance at rated power and boost available at no or low load conditions. For example, engine operation at rated power requires a large frame size turbine and compressor, which can effectively draw power from the engine's exhaust gas and use a portion of that power to compress air provided to the engine for combustion. At rated power, the engine will burn a large amount of fuel, which requires a large amount of air to maintain a desired air to fuel ratio. The relatively large turbocharger frame size can provide the appropriate air amount at a relatively low engine pumping loss, which increases engine efficiency. However, while engine operation at the rated power tends to require a large turbocharger frame size, as discussed above, low or no load engine performance, and also engine transient performance when the engine is accelerating, tends to require a smaller frame size. This is because the rotational inertia of the turbine and compressor wheel, and also the pressure that can be built up in the turbine and compressor, and also the rate at which pressure can be built up, will be improved from a smaller frame size turbocharger, which will also include smaller turbine and compressor wheels having a smaller rotational inertia permitting them to accelerate faster. The effect of a delay in engine power output increase because of the turbocharger is sometimes referred to as turbocharger or turbine lag or delay.
Thus, a tradeoff exists when sizing a turbocharger for an internal combustion engine. Typically, engines use a relatively larger frame size turbocharger to achieve desired operational characteristics at high engine loads, which compromises low end engine performance and can increase turbocharger lag. In an engine having a larger frame size turbocharger, at low or no engine load conditions such as idle, accelerating engine performance, snap torque increases and the like will be affected and limited by the amount of air that the turbocharger can provide if the engine is to maintain a desired air/fuel ratio.
Various solutions have been proposed in the past to address issues with turbocharger lag. For example, certain engines may use more than one turbocharger that operate in sequence or in parallel. In such engines, a large turbocharger may be used primarily for high load conditions while a second, smaller turbocharger may be used at low or no load conditions. These differently sized turbochargers, however, may still not address quick power increases of the engine output when the engine is operating at a mid-range condition. Other engines use two turbochargers in parallel, each of which is sized for a mid-engine range. These engines too, however, may not effectively and efficiently operate at rated power and no or low load conditions, and they may also increase engine pumping losses by the additional exhaust and fresh air conduits that are required to connect them to the engine.
An additional solution proposed in the past to in an attempt to decrease the effects of turbocharger lag in engine performance is to use a turbine having a variable frame. While a turbine of this type can simulate a smaller frame turbine, which can help in the low end of engine performance, the compressor is typically a fixed-frame compressor that is sized for the high end of engine performance, so the solution is only partly suited to address transient engine performance.